Searching for oil or more generally hydrocarbons is becoming more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper or in positions difficult to reach. Prospecting and exploiting hydrocarbon fields laying under deep sea has become customary and necessitates hardware which is more resistant to environmental challenges like fatigue and corrosion, which were previously less important.
For extracting oil or gas from fields lying below deep sea off-shore platforms are generally used which are anchored to the sea bottom and tube strings are used which are conventionally called risers. They serve to convey oil or gas to the surface.
These tube strings are immersed in the sea and are subject to movements caused by sea streams and surface wave movements. Because of these continuous and periodic movements of the sea the risers do not remain immobile, but are subject to lateral movements of small magnitude which can cause deformations in certain parts of the joint and must withstand loads which induce fatigue stresses in the tubes, with particular respect in the zone of the threaded joint. These stresses tend to cause ruptures in the tubes in the vicinity of the thread and there is a need to improve the fatigue resistance of the threaded joints.
At present, fatigue performance and design for threaded connections for the oil and gas industry are being adapted and extrapolated from other engineering fields. There are no specific standards or design/dimensioning specifications yet. Basic concepts can be found in the British Standard/Code of practice BS7608 for Fatigue design and assessment of steel structures, and DNV Class B S-N curve. Solutions have already been proposed to increase fatigue life of the threaded joints.
U.S. Pat. No. 6,045,165 discloses a method for manufacturing and for making up a threaded joint to improve its corrosion resistance. The joint has either an internal shoulder or an external shoulder or both, each comprising a seal portion and a torque shoulder. It is designed in such a manner that the pitch diameter of the seal forming face on the pin is larger than the same on the box and the inclination of the torque shoulder forming face on the pin is smaller than the same on the box. The difference in the inclination between both sides is in the range of 0.5° to 4.0°. These features provide gas tightness and crevice corrosion resistance.
However the problem of fatigue resistance has not been dealt with in that document.